The Phoenix Zoo's Linda Hardwick talks about how both the animal residents and visitors beat the heat on June 19, 2017. Mark Henle/azcentral.com

Some flights involving small regional jets have been canceled at Sky Harbor because of the heat. Larger planes are not likely to be affected.(Photo: Michael Chow/The Republic)

PHOENIX — The extreme heat forecast for Phoenix on Tuesday has caused the cancellation of 20 American Airlines flights out of Sky Harbor International Airport.

According to a statement from American Airlines, the American Eagle regional flights use the Bombardier CRJ aircraft, which has a maximum operating temperature of 118 degrees. Tuesday's forecast for Phoenix included a high of 120 degrees, and the flights that are affected were to take off between 3 and 6 p.m. MT.

Customers affected were told to contact American Airlines for rebooking options or to request a refund. Extreme heat affects a plane's ability to take off. Hot air is less dense than cold air, and the hotter the temperature, the more speed a plane needs to lift off. A runway might not be long enough to allow a plane to achieve the necessary extra speed.

American Airlines alerted travelers on Saturday about the heat wave and recommended they change any flights scheduled to arrive or depart between 3 and 6 p.m. Monday, Tuesday or Wednesday. The flight changes would be free of charge.

This is reminiscent of Phoenix's record-setting high temperature of 122 degrees on June 26, 1990, which grounded some airlines for the day. Larger jets, such as Airbus and Boeing, aren't expected to be affected by this week's heat.